String trimmers are well-known devices for yard and lawn maintenance that may be used to easily accomplish tasks that may be difficult for other devices, such as trimming plants near a sidewalk or a wall. A number of types of string trimmers are known in the art; for example, some designs include a battery and an electric motor to provide rotary motion for trimming plants. As another example, other designs include an internal combustion engine to provide rotary motion.
In any case, most string trimmer designs include a power source (e.g., the electric motor or internal combustion engine) that connects through an elongated transmission assembly to a rotary whip assembly that engages and trims plants. The power source and the rotary whip assembly are positioned at opposite ends of the transmission assembly to distribute the weight of the trimmer and reduce the amount of torque that must be applied by a user to hold the trimmer. In addition, the transmission assembly typically has a curved or bent shape to provide a design in which the rotary assembly can be positioned away from the user and oriented to effectively trim plants.
The transmission assembly of a typical string trimmer includes a hollow outer tube, or a “down-tube”, and a flexible plastic liner that generally centers a rotatable drive shaft, or a “core”, within the down-tube. A core is typically constructed of multiple helically wound metal wires to provide flexibility. In some designs, the flexibility of the core permits the core to bend to follow the curve of the down-tube and directly connect the power source to the rotary whip assembly. In other designs, multiple liners and cores are housed in a single down-tube and the cores connect to one another at the bend of the down-tube.
However, the transmission assemblies of current string trimmer designs include several drawbacks. For example, the core needs to be accurately centered at certain locations along the length of the down-tube, such as interfaces with other components, to prevent excessive wear. However, a typical liner may not be able to accurately center a core due to the inherent flexibility of the liner, particularly where the core exits the down-tube, and especially at the lower end of the down-tube if the down-tube is curved at the lower end. As another example, rotation of the helical core may cause a “threading” and “unthreading” motion against the liner and the liner may shift axially within the down-tube. This motion may cause the liner to wear and reduce its effectiveness for generally centering the core.
Considering the limitations of current transmission assemblies for string trimmers, a need exists for an assembly with an accurately centered core in at least some positions within the down-tube. A need also exists for such an assembly to restrain the flexible liner relative to the down-tube. Further still, a need also exists for such an assembly that is easily and inexpensively manufactured.